1. Field of the Invention
This invention relates to a .beta.-galactosidase, including an SDS-resistant .beta.-galactosidase, which has a hyperthermostability and is useful in the food industry and a process for producing the same.
The invention also relates to a gene coding for the hyperthermostable .beta.-galactosidase and a genetic engineering process for producing the enzyme which is useful in the fields of, for example, food industry and saccharide engineering.
2. Description of Related Art
.beta.-galactosidase, which is an enzyme capable of decomposing .beta.-galactoside, has been discovered in animals, plants and microorganisms. It is known that this enzyme occurs particularly in bacteria such as Escherichia coli, Streptococcus lactis, Bacillus subtillis, Streptococcus thermophilus and Sulfolobus solfataricus. Such a .beta.-galactosidase in applied to the production of low-lactose milk by taking advantage of its ability to hydrolyze lactose into galactose and glucose. It is also applied to the production of galactose or glucose from lactose contained in milk serum which is formed in a large amount in the processor producing cheese.
To apply a .beta.-galactosidase to food processing, therefore, it has been demanded to develop an enzyme that can withstand its use at a high temperature from the viewpoint of preventing contamination with microorganisms during processing and from another viewpoint of elevating the solubility of lactose which serves as a substrate.
Further, in recent years, various sugar compound productions are conducted with the use of a .beta.-galactosidase glycosyltransfer reaction Japanese Patent Laid-Open No. 25275/1994 and Japanese Patent Laid-Open 14774/1994!. Thus, the development of highly thermostable enzymes is desired.
For example, a .beta.-galactosidase originating in Sulfolobus solfataricus see European Journal of Biochemistry, 187, 321-328 (1990)! is a thermophilic enzyme having an activity at a temperature of 90.degree. C. However, its activity falls to about 50% after treating at 85.degree. C. for 180 minutes.
It is described in, for example, European Journal of Biochemistry, 213, 305-312 (1993), that .beta.-galactosidase derived from the hyperthermophilic bacterium Pyrococcus furiosus exhibits its activity at high temperatures, thereby ensuring a high thermostability.
The inventors have discovered a hyperthermostable .beta.-galactosidase having a residual activity ratio of about 80% even after treatment at 90.degree. C. for 120 minutes and succeeded in isolating three types of .beta.-galactosidases European Patent Laid-Open No. 0592158A2!. These three types of .beta.-galactosidases are all hyperthemostable, and one of them is a .beta.-galactosidase having an extremely high stability and exhibits its activity even in the presence of 1% sodium dodecyl sulfate (SDS).